I guess I'm a biased KDE user, but I prefer KDE apps in many or most instances. As another commenter noted, Gwenview is stable, fast, and reasonably powerful. As for photo-editing apps, most people may prefer Gimp, but I think Krita can hold a candle and even has a few features that Gimp doesn't (see this comparison).
Other examples of (in my opinion) superior KDE apps include Dolphin (vs Nautilus), Kate (vs Gedit), Kile (a LaTeX IDE, Gnome has nothing comparible), Kmail (vs Evolution), Okular (vs Evince), and K3B (vs Brasero).
There are definitely some Gnome apps that I find better as well, including Inkscape (vs. Karbon) and the newsgroup app you mentioned, Pan. I should add that not all of these are really "Gnome" apps, but they all use GTK.
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